It is desirable during the production of oil and gas to carry out flow measurements to determine the flow rates of individual phases of multiphase flow. In particular, measurement of the volume fractions and flow velocities of e.g. oil, gas and water in a conduit, such as a pipe, is highly desirable.
However, in general it is very difficult to obtain measurements of the flow of the different phases when they flow simultaneously through a pipe.
This difficulty is primarily due to the wide variety of flow regimes such a multiphase flow can take. For example, the three phases can be well mixed together with one as the continuous phase and the other two dispersed within it. Mostly there is phase separation between gas and liquid with the liquid often moving at a much lower velocity than the gas.
When gas is the dominant phase, a commonly encountered flow regime in a vertical pipe is for the gas to travel along the centre of the pipe with dispersed droplets of oil and water within it, whilst the majority of the oil and water travels along the pipe wall which itself may comprise entrained gas bubbles.
Additionally, flow phase and velocity distributions may alter both spatially and temporally. Sudden or gradual variation in flow rates of one phase or another may cause a change in flow regime. Also, due to the high pressure encountered deep underground, a flow which is mixed or in bubble-flow can become dominated by a discernible high gas fraction as the pressure drops nearer the surface and the gas expands and/or comes out of solution.
Multiphase flowmeters are available and have been suggested in the prior art.
The use of electromagnetic methods, such as microwaves, has been suggested because of their high measurement sensitivity to the presence of the water phase in a multiphase flow (water permittivity/conductivity is much higher than the permittivity/conductivity of the hydrocarbon oil-gas phases). U.S. Pat. No. 6,831,470 of the Applicant teaches the use of a microwave open-ended coaxial reflection probe to measure the mixture permittivity and mixture conductivity to obtain an online estimate of water conductivity of a multiphase flow. An estimate of the water-to-liquid ratio (WLR) immune to water-salinity change is also possible if the liquid layer in the vicinity of the probe is substantially free of entrained gas and has a thickness higher than the probe's depth of investigation.
U.S. Pat. No. 7,908,930 of the Applicant utilises a transmission electromagnetic approach, in combination with a venturi differential-pressure sensor (for total flow rate) and a gamma-ray radiation sensor (for gas-liquid mixture density). The across-pipe transmission microwaves are used to measure the mixture permittivity and mixture conductivity over the vertical pipe cross-section of the venturi throat, for water and hydrocarbon (oil/gas) discrimination. The gamma rays are employed in the same venturi-throat pipe cross-section for gas and liquid (oil/water) discrimination, by measuring the average fluid mixture density across pipe. By employing three-phase density and permittivity and/or conductivity mixing rules, measures of water fraction, oil fraction (hence of the WLR) and gas fraction can be obtained. Measured venturi differential pressure and/or further microwave sensors in the venturi for flow velocity can be used to provide an estimate of the individual phase flow rate, from the measured individual phase fraction and the total flow rate and/or the phase velocity data. As disclosed in U.S. Pat. No. 6,831,470, RF/microwave transmission approach also permits online water-conductivity estimate from the measured mixture permittivity and mixture conductivity.
U.S. Pat. No. 5,485,743 of the Applicant discloses a method for measuring multiphase flows in a pipe using an array (e.g. twelve) of microwave antennas arranged around the pipe. Each antenna is capable of transmitting microwave energy (at one or more frequencies) into the pipe and detecting propagated microwave energy in the pipe. Microwave energy from each antenna is transmitted in turn while the propagated microwave energy is detected at the non-transmitting antennas so as to generate multiple amplitude-attenuation and phase-shift output signals. The output signals from all antennas combinations are interpreted by an appropriate mathematical inversion algorithm, e.g. as flow permittivity-conductivity cross-sectional/tomographic images, so as to measure the flow phase fractions and to visualise flow phase distributions in the pipe. Only absolute measurements are disclosed and no differential-measurement scheme (of measuring amplitude-attenuation ratio and phase-shift difference of chosen two receivers, with respect to one chosen transmitter) is mentioned.
U.S. Pat. No. 7,624,652 discloses a differential-measurement configuration based on one transmitter and two receivers, where the amplitude-attenuation ratio and/or phase-shift difference of the two receivers, measured at multiple frequencies with respect to a chosen transmitter, are used for flow-mixture dielectric-constant determination.
In all of these electromagnetic methods, a measure of the permittivity and conductivity of the flow mixture is involved by analysing phase-shift and amplitude-attenuation. Permittivity and/or conductivity data allows a host of useful flow information within the conduit to be obtained, such as water conductivity, water fraction, WLR, flow rates of individual phases, in combination with a differential pressure and a nuclear mixture-density measurement; information as to the distribution of fluid phases within the conduit can be obtained from measurements of a plurality of RF/microwave antennas arranged around the conduit.